Devices for injecting preheated air in a shaft furnace, more generally known under the name "tuyere-stock", are subject to problems of mobility and sealing. In fact, as a result of the high temperature of the preheated air (a temperature on the order of 1200.degree. C. or more) and of the high temperature prevailing inside the furnace, the wall of the latter and the bustle pipe and the tuyere-stock are subject to thermal expansions and deformations which cause significant relative displacements between the bustle pipe and the wall of the furnace. The tuyere-stock must therefore be capable of compensating for these relative displacements, while at the same time preventing leaks of gas or preheated air.
To meet these requirements, U.S. Pat. No. 3,766,868 discloses a tuyere-stock of the type employing separate cylindrical members with an intermediate member being connected to the end members by means of articulated joints comprised of cardan compensators. This tuyere-stock has since been improved by the design of universal ball-and-socket joints like those described in German patent specification DE-C2-2218331. The three joints of this tuyere-stock make it possible to compensate all the relative movements between the bustle pipe and the wall of the furnace. Sealing in the region of the joints is obtained by means of bellows expansion joints, while mechanical stability is ensured by means of cardan connections associated with the two opposite ends of the central tubular element in the region of the two universal joints.
It will be appreciated that the most stressed and most critical part of the tuyere-stock is always located in the place of the joints. In fact, the mobility of the ball portion with respect to its socket often leans to irreversible distortions of the soft joints and to frictions among refractory materials. In addition, given the difficulties of machining refractory steel, it is not possible to extend the amouring, forming the casing of reason why microcracks are often formed in the refractory of the tip of the ball portion, causing disturbing circulations and whirls.
In addition to these quality criteria for a reliable tuyere-stock are added the concern for a competitive manufacturing price, for the possibility of easy and rapid dismounting, for the possibility to easily renew or replace the refractory material if necessary, etc. It will be appreciated that all those criteria often tend to oppose each other, thus forcing the designer and manufacturer to choose a reasonable compromise.